Tamperproof retractable syringe

Abstract

A tamperproof retractable non-reusable syringe has a one piece hollow outer body with a barrel for a slidable plunger, a transition zone and a smaller diameter nose portion. An elongated needle holder and spring combination is installable from the rear of the outer body, guided into the nose portion and held by cooperating inwardly and outwardly facing surfaces oriented in the direction of retraction at the most constricted past of the transition zone where the nose begins. The plunger has an opening with a dislodgable stopper for receiving parts of the retraction mechanism. The stopper and the head of the needle holder are of significantly reduced diameter from the injection fluid chamber to resist blowing out prematurely. In one embodiment the head of the needle holder is surrounded by a separable retainer member which is slidingly removed by contact with the tip of the plunger after the stopper is mostly or fully removed to avoid cumulation of force required for retraction after the injection. In a second embodiment the head of the needle holder is clamped and held by constricting forces imposed by stress on the outer body induced by interference fit. Release occurs by slight expansion on the barrel by contact of the plunger tip with a small internal ramp in the outer barrel. Both embodiments have a plunger cap configured to enter an opening in the outer body to provide an additional tamperproof feature.

I claim: 
     
       1. A tamperproof retractable syringe for injecting fluid wherein the syringe has a one piece body and a frictionally held retraction mechanism assembleable from the rear which is retractable with low plunger force and resists high blowout pressure during an injection, comprising: a one piece hollow outer body having a longitudinally extending wall, comprising an elongated barrel and nose, with a transition zone connecting the barrel and nose, the nose having a reduced cross sectional area relative to the barrel and an inwardly facing surface in the wall at the most constricted part of the transition zone where the nose begins;   a plunger assembly disposed partially within the elongated barrel, the plunger having a head in slidable sealed contact with the interior of the outer body and a retraction cavity therein for receiving parts of a retraction mechanism;   a retraction mechanism comprising an elongated needle holder and spring combination wherein the needle holder is released by depression of the plunger to a retraction position, the needle holder having an elongated body with a needle holding portion in front and a head in back, having a cooperating outwardly facing surface configured to cooperate with said inwardly facing surface to produce a holding force on the needle holder when the retraction mechanism is installed in the nose;   the needle holder and spring being installable from the rear of the barrel toward the nose and releaseably held by sliding engagement of said inwardly and outwardly facing surfaces while compressing said spring, said sliding engagement producing said holding force in opposition to a retraction force applied to the needle holder by said spring;   retraction occurring in response to thumb force on the plunger when a portion of said plunger passing into said transition zone causes the cooperating outwardly facing surface to slide relative to said inwardly facing surface and thereby reduces the holding force on the needle holder to an amount less than the retraction force whereby the needle holder is retracted into said cavity a distance sufficient to withdraw an injection needle into the outer body.   
     
     
       2. The tamperproof retractable syringe of claim 1 wherein the inwardly facing surface in the wall and the cooperating outwardly facing surface on the needle holder are friction surfaces which cooperate to produce said holding force on said needle holder as a frictional holding force. 
     
     
       3. The tamperproof retractable syringe of claim 2 wherein said frictional holding surfaces comprise a linear interface aligned in the direction of retraction. 
     
     
       4. The tamperproof retractable syringe of claim 1 wherein the plunger has a graspable end cap for depressing the plunger and a length selected to allow the end cap to enter an opening of the barrel when the plunger is depressed to the retraction position, in order to prevent tampering after retraction. 
     
     
       5. The tamperproof retractable syringe of claim 1 wherein the needle holder and the most constricted part of the transition zone where the nose begins comprise a lower boundary of a variable fluid chamber below the plunger head and the needle holder has a fluid path for injection fluid opening into the variable chamber. 
     
     
       6. The tamperproof retractable syringe of claim 5 wherein the plunger head has a tip with an opening sealingly closed by a dislodgeable held stopper which slides relative to the plunger in response to dislodging force applied by depression of the plunger at the end of the injection cycle before retraction occurs. 
     
     
       7. The tamperproof retractable syringe of claim 6 wherein at the end of the injection cycle a forward end of the dislodgeable stopper comes in contact with the needle holder to block said fluid path whereby said dislodging force is applied during continued depression of the plunger prior to retraction. 
     
     
       8. The tamperproof retractable syringe of claim 7 wherein the ratio of the greatest cross sectional areas of the variable chamber and the dislodgeable stopper is selected so that the maximum expected thumb force on the plunger during an injection will produce a maximum pressure in the chamber which will generate a force on the stopper slightly less than the amount of dislodging force necessary to dislodge the stopper so that the stopper will not blowout during an injection. 
     
     
       9. The tamperproof retractable syringe of clam 8 wherein said ratio of the area of the variable chamber to the area of the dislodgeable stopper is at least two to one so that at least twice the force necessary to dislodge the stopper can be applied to the plunger during an injection without blowout of the stopper. 
     
     
       10. The tamperproof retractable syringe of claim 6 wherein said nose has a wall defining an internal cavity which closely confines the spring and needle holder combination and serves as a guide to steer the needle holder and uncompressed spring into a compressed state of the spring to facilitate assembly from the rear without jamming. 
     
     
       11. The tamperproof retractable syringe of claim 10 wherein the front of the nose is provided with an opening and the front needle holding portion of the needle holder extends outwardly through said opening in order to facilitate installation of a needle from the front of the syringe after the needle holder has already been assembled in the nose. 
     
     
       12. The tamperproof retractable syringe of claim 1 wherein the head of the needle holder is coupled to a separable retainer member, along a sliding interface oriented in the direction of retraction, with a holding force which exceeds said retraction force, the outer surface of said retainer member having said outwardly facing surface configured to cooperate with said inwardly facing surface to produce said holding force on the needle holder when the retraction mechanism is installed in the nose. 
     
     
       13. The tamperproof retractable syringe of claim 12 wherein said ratio of the greatest cross sectional area of the variable chamber to the area of the retainer member exposed to fluid is selected so that the retainer member has a high blowout pressure higher than the maximum pressure expected, as a result of maximum expected thumb force on the plunger and a low frictional holding force to be overcome during retraction so that the thumb force required to trigger retraction is comfortably and substantially lower than said maximum thumb force expected. 
     
     
       14. The tamperproof retractable syringe of claim 13 wherein the ratio of the greatest cross sectional area of the variable chamber to the cross sectional area of the retainer member exposed to fluid in the variable chamber is not less than about two to one so that high blowout pressure can be resisted while retaining a relatively low force on the plunger necessary to cause retraction. 
     
     
       15. The tamperproof retractable syringe of claim 2 wherein the head of the needle holder is a two part head comprising a head surrounded by a separable retainer member coupled with a friction force which exceeds said retraction force along a sliding interface oriented in the direction of retraction, the outer surface of said retainer member being said outwardly facing friction surface which cooperates with said inwardly facing friction surface to retain said needle holder in an unretracted position at the most constricted portion of the transition zone where the nose begins. 
     
     
       16. The tamperproof retractable syringe of claim 15 wherein the body of the needle holder is grounded in the nose portion against forward movement and the front of the plunger head is configured to pass through the said most constricted area and push against said retainer member without also pushing against the head of said needle holder and thereby cause retraction. 
     
     
       17. The tamperproof syringe of claim 16 wherein the frictional holding force on the needle holder to be overcome by thumb force on the plunger during retraction is relatively low relative to the maximum expected force on the plunger during an injection, the retainer member having a reduced surface area exposed to fluid pressure in the variable chamber selected to resist blowout and premature retraction. 
     
     
       18. The tamperproof syringe of claim 17 wherein the ratio of the cross sectional area of the variable chamber to said reduced surface area of the retainer member exposed to fluid pressure in the variable chamber is not less than about two to one. 
     
     
       19. The tamperproof retractable syringe of claim 16 wherein the needle holder and the most constricted part of the transition zone where the nose begins comprise a lower boundary of a variable fluid chamber below the plunger head and the needle holder has a fluid path for injection fluid opening into the variable chamber. 
     
     
       20. The tamperproof retractable syringe of claim 19 wherein the plunger head has an opening sealingly closed by a dislodgeable frictionally held stopper which slides relative to the plunger in response to dislodging force applied by depression of the plunger at the end of the injection cycle before retraction occurs. 
     
     
       21. The tamperproof retractable syringe of claim 20 wherein the greatest cross sectional areas of the variable chamber and the dislodgeable stopper are selected so that the maximum expected thumb force on the plunger during an injection will produce a maximum pressure which will generate a force on the stopper slightly less than the amount of dislodging force necessary to dislodge the stopper so that the stopper will not blowout during an injection. 
     
     
       22. A tamperproof retractable syringe structure for injecting fluid into a patient comprising: a syringe body having a wall forming an elongated barrel portion with a smaller nose portion in front and a transition zone between the barrel portion and the nose portion;   a moveable plunger in the barrel portion having a front end and a back end, the plunger having a head at the front end in sliding sealed contact with the interior of the barrel, a cap at the back end for applying thumb force to the plunger, and a cavity for receiving retractable parts;   a retraction mechanism disposed in the nose portion of the syringe body having retractable parts comprising a releasable needle holder and needle frictionally held by the wall of the syringe body with the needle extended from the nose portion, a biasing element applying a retraction force to the needle holder and a fluid path traversing the needle and needle holder;   the head of the plunger having an opening into said cavity, sized to receive the retractable parts and a releasable stopper extending from said opening, the stopper sealing the interior of the plunger from injection fluid stored in a variable chamber defined in the barrel between the retraction mechanism and the head of the plunger;   the plunger being depressible to a first position to expel injection fluid from the variable chamber through the fluid path in response to thumb pressure on said cap, said first position comprising the end of an injection;   said plunger being further depressible to a retraction position beyond the first position whereby said stopper is dislodged and said releasable needle holder is released from the syringe body and retracted into the cavity of the plunger a distance sufficient to withdraw said needle entirely within the syringe body;   said plunger being a length selected to remain graspable behind the barrel portion of the syringe body in the first position of the plunger and become ungraspable by withdrawal of the periphery of the cap within the syringe body in the second position of the plunger so that the retracted syringe cannot be tampered with.   
     
     
       23. The tamperproof retractable syringe of claim 22 wherein said syringe body is a one piece hollow body wherein said retraction mechanism is installable from the rear of the barrel toward the nose and releasably held by sliding engagement of cooperating inwardly and outwardly facing surfaces while compressing said spring. 
     
     
       24. The tamperproof retractable syringe of claim 23 wherein the nose closely confines the retractable parts comprising the needle holder and biasing element and serves to guide and steer them into position in the nose. 
     
     
       25. The tamperproof retractable syringe of claim 24 wherein the needle holder has an inner head and a separable :retainer member surrounding the inner head coupled along a sliding interface oriented in the retraction direction, said cooperating outwardly facing surface being the outer surface of said retainer member whereby said plunger is depressible to release said needle holder by sliding separation of said retainer member. 
     
     
       26. The tamperproof retractable syringe of claim 25 wherein the ratio of the greatest cross sectional area of the variable chamber to the area of the retainer member and to the area of the stopper exposed to fluid in the variable chamber are selected so that the maximum expected thumb force on the plunger during an injection will produce a pressure force on the stopper and retainer slightly less than the amount necessary to dislodge them so that they will not blowout during an injection. 
     
     
       27. The tamperproof syringe of claim 26 wherein said ratios are not less than two to one. 
     
     
       28. A tamperproof retractable syringe structure having low retraction force comprising: a hollow syringe body having an elongated barrel portion extending between a nose portion adapted to support a needle holder and a rear portion having an opening for receiving a plunger from the back of the body;   an elongated plunger having a leading head end in front and a trailing back end provided with an end cap for pressing, the head end having an opening in front leading to a hollow cavity in the plunger extending rearwardly toward the back end, the head end being receivable through the opening in the rear portion of the syringe body and being movable in sliding sealed contact with the interior of the barrel;   a frictionally held dislodgeable stopper extending in sealing relation from the opening in the head end of the plunger whereby an upper boundary of a variable chamber for injection fluid is formed below the head of the plunger in the barrel of the syringe;   a needle holder and needle releaseably mounted in the nose portion of the syringe body with the needle extended, the needle holder having a elongated stem portion with a front end grounded against forward motion and a spaced apart enlarged head being releasably held in the nose portion of the syringe body by means of frictional force caused by constricting forces imposed thereon by the nose portion of the syringe body, whereby a lower boundary of said variable chamber is formed above the enlarged head of the needle holder;   the needle holder and needle defining a fluid path for injection fluid opening into the variable chamber, said needle holder being sized to retract through said opening in the head end of the plunger;   a biasing element configured to apply a retraction force to the needle holder in said nose portion which is less than said frictional force, said biasing element having sufficient travel when unrestrained to carry the needle holder into the cavity of the plunger a distance sufficient to withdraw the extended needle within the syringe body when the needle holder is released from the nose portion of the syringe body;   the plunger being depressible in an injection stroke to expel injection fluid from the variable chamber until the plunger reaches a first position comprising the end of an injection stroke wherein the needle holder is encountered by the stopper;   said plunger being further depressible toward a retraction position whereby said stopper is being dislodged from said opening, the plunger reaching an intermediate position beyond said first position whereby further forward movement is effective to gradually remove the constricting forces holding the needle holder until said retraction position of the plunger is reached wherein said frictional force holding the needle holder becomes less than said retraction force applied by the biasing element and said needle holder is thereby released from said nose portion and retracted into said cavity.   
     
     
       29. The tamperproof syringe of claim 28 wherein the plunger length is selected to remain graspable by means of the end cap at the trailing end when the plunger is depressed to said first position, the plunger upon further depression to said second position, becoming ungraspable by passage of the end cap into the rear portion of the syringe body so that the retracted syringe cannot be tampered with. 
     
     
       30. The tamperproof syringe of claim 29 wherein the biasing element is a spring loosely surrounding the stem portion of the needle holder in close proximity thereof, the spring having a front end restrained by the nose portion of the syringe body and a rear end under the enlarged head of the needle holder. 
     
     
       31. The tamperproof syringe of claim 30 wherein said nose portion has a wall defining an internal cavity extended in the direction of retraction which closely confines the spring and thereby facilitates assembling from the rear the needle holder and uncompressed spring into a compressed state in said nose portion. 
     
     
       32. The tamperproof syringe of claim 32 wherein an opening is provided in the front of the nose portion of the syringe body and a part of the stem portion of the needle holder extends outwardly through said opening in order to facilitate installation of the needle into the stem portion of the needle holder from the front of the syringe after the needle holder has already been assembled in the nose portion of the syringe body. 
     
     
       33. The tamperproof syringe of claim 28 wherein the ratio of the cross sectional areas of the variable chamber and the dislodgeable stopper is selected so that the maximum expected thumb force on the plunger during an injection will produce a pressure force which is slightly less than the amount of force necessary to dislodge the stopper or cause it to move significantly with respect to the plunger opening. 
     
     
       34. The tamperproof retractable syringe of claim 33 wherein said ratio of the cross sectional areas of the variable chamber to the area of the dislodgeable stopper is at least two to one so that at least twice the force to dislodge the stopper can be applied to the plunger during an injection without blowout of the stopper.